1. Field of the Invention
The present invention relates to a plasma display apparatus and image processing method thereof.
2. Background of the Related Art
Generally, in a plasma display panel (hereinafter, referred to as “PDP”), a barrier rib formed between a front glass and a rear glass, which are made of soda-lime glass, forms one unit cell. Each cell is filled with a main discharge gas, such as neon (Ne), helium (He) or a mixed gas (Ne+He) of Ne and He, and an inert gas containing a small amount of xenon. When the PDP is discharged by a high frequency voltage, the inert gas generates vacuum ultraviolet rays, and light-emits phosphors formed between the barrier ribs to implement an image.
Such a PDP can be easily fabricated since it has a simple structure compared to a cathode ray tube (CRT) that has been mainly used as the display means. Further, the PDP has characteristics that it can be made thin and large compared to the CRT, and has been spotlighted as a next-generation display apparatus.
FIG. 1 is a view for explaining gray level representation of a conventional PDP. As shown in FIG. 1, the three-electrode AC surface discharge type PDP is driven with one frame being divided into several sub-fields having a different number of emission in order to implement gray levels of an image.
Each of the sub fields is subdivided into a reset period for uniformly generating discharging, an address period for selecting a discharge cell, and a sustain period for implementing gray levels depending on the number of discharging of a sustain pulse. For example, if it is desired to display an image with 256 gray levels, a frame period (16.67 ms) corresponding to 1/60 seconds is divided into eight sub-fields SF1 to SF8, as shown in FIG. 1.
Each of the eight sub-fields SF1 to SF8 is subdivided into a reset period, an address period and a sustain period. At this time, the reset period and the address period of each of the sub-fields are the same every sub-field, but the sustain period of each of the sub-fields and the number of discharging of a sustain pulse increase in the ratio of 2n (n=0, 1, 2, 3, 4, 5, 6, 7) in each sub-field. As such, as the sustain period is different in each sub-field, gray levels of an image can be represented.
FIG. 2 is a graph showing comparison results of brightness characteristics between a PDP and a CRT. As shown in FIG. 2, the CRT and LCD represent a desired gray level by controlling displayed light in analog mode according to an input video signal. Thus, they have non-linear brightness characteristics. In contrast, the PDP represents a gray level by modulating the number of light pulses using a matrix array of a discharge cell that can be turned on or off. It thus has linear brightness characteristics.
This method of representing the gray level of the PDP is called a “pulse width modulation (PWM) method”. The brightness of the PDP varies linearly against the number of pulses. As the degree that is recognized by the naked eyes is non-linear, however, noise is generated when the gray level is represented in a low gray level region. Accordingly, in order to solve this problem, input video data undergo inverse gamma correction in the conventional PDP. That is, after a reference brightness value such as the CRT brightness curve of FIG. 2 is set, a gamma curve data LUT storage unit in which gamma curve data look-up table (LUT) corresponding to reference brightness values are stored is provided, and input gray level values undergo inverse gamma correction.
Meanwhile, the results of measuring real brightness values depending upon gray level values before the inverse gamma correction is shown in FIG. 3.
FIG. 3a is a table showing a mapping state of sub-fields depending upon gray level values of the conventional PDP. FIG. 3b is a graph showing the relationship between a real brightness value and a gray level value of the conventional PDP.
From FIG. 3a, it can be seen that when a gray level is represented by turning on/off a sub-field, the number of address discharge (a), which is additionally required to individually control each sub-field, as well as the sustain discharge (s) depending upon sub-field weight for representing the gray levels is different. That is, since light by a reset discharge (r) and the address discharge (a), which are additionally needed to individually control each sub-field, is displayed on a screen, a real brightness value depending upon a gray level value before inverse gamma correction does not increase linearly, as shown in FIG. 3b. 
As such, a phenomenon in which a gray level having a lower gray level value, among neighboring gray levels, has a higher brightness value since it has a greater number of reset discharge and address discharge, is called an “inversion phenomenon of a gray level”. In this case, there occurs a problem in that the linearity of a gray level for inverse gamma correction is not secured due to the inversion phenomenon.